The document number is usually automatically generated by the system and determined by the document type of the document. If the document is not saved, the preliminary number is displayed in "<>". If the document type of your document has no automatic document sequence defined, the field will be empty when creating a new document. This is for documents which usually have an external number (like vendor invoice). If you leave the field empty, the system will generate a document number for you. The document sequence used for this fallback number is defined in the "Document Sequence" window with the name "DocumentNo_<TableName>", where TableName is the actual name of the table inside the database (e.g. C_Order).